1. Field of the Invention
The present invention relates to a voltage controlled PWM (Pulse Width Modulated) frequency converter comprising a single phase rectifier bridge, a DC intermediate circuit and a controlled inverter bridge for generating an AC output voltage with varying voltage and frequency. The present invention also relates to a method for controlling such a frequency converter.
2. Description of Background Art
FIG. 1a presents a prior art single phase PWM frequency converter. It comprises a rectifier bridge 10 for rectifying an AC voltage Uin of a supply line to a DC voltage Udc for a DC intermediate circuit 11 and an inverter bridge 12 for the inversion of the intermediate circuit DC voltage Udc into a single phase or three phase variable frequency AC voltage Uout. Such single phase frequency converter may be connected to small single or three phase AC loads, such as a pump or fan motor 13. The inverter bridge 12 may be a full-wave bridge with pulse-width-modulated semiconductor switches V11 to V16, such as insulated gate bipolar transistor (IGBTs), and flywheel diodes D11 to D16 connected in inverse-parallel with the semiconductor switches. The semiconductor switches V11 to V16 are controlled with pulse-width modulation by means of a PWM control unit 14. The rectifier bridge 10 may be a full-wave diode bridge with four diodes D1 to D4 connected to the phase and neutral lines of the AC supply voltage Uin.
FIG. 1b presents some typical signal waveforms of a prior art single phase converter. The target of the motor control is normally to get the motor shaft torque to be as constant as possible in a constant operation point. For this target it is good if the DC intermediate circuit voltage is constant, because then the formation of the exact output voltage Uout which is essential in determining the motor shaft torque, is easier. This is why the capacitance of the DC intermediate circuit capacitor Cdc is normally dimensioned to be very large. The DC intermediate circuit current Idc consists of pulses according to the inverter PWM operation. When the voltage Udc is constant and the motor operation point stable, the pulse train Idc is quite constant, as shown in FIG. 1b. 
The smooth DC intermediate circuit voltage Udc causes the rectified supply AC voltage Uin to be higher than the DC voltage Udc only for very short periods. This causes the supply phase current waveform Iin to be very narrow and high pulse according to FIG. 1b, because the current Iin can flow only when Uin is higher than Udc. This kind of line current waveform causes problems; e.g., in component dimensioning, and it can cause electric noise problems in the supply line.
There are several known methods to reduce the supply line current problem. Extra filtering, consisting of reactors and capacitors, can be used. One known solution is the so-called PFC (Power Factor Correction) circuit shown in FIG. 2a. It consists of a reactor L21, a diode D21 and a semiconductor switch V21 such as an IGBT. The switch V21 is controlled so that the reactor current is as close as possible sinusoidal and in phase with the line voltage Uin (see FIG. 2b). When using PFC, the DC intermediate circuit voltage Udc is normally constant and higher than the peak value of the line voltage Uin. EP-A2-1170853 discloses a single-phase AC-DC converter including a PFC power supply section, where a rectified current obtained by rectifying an electric current from an AC supply is switched, a DC—DC power supply section, where a direct current obtained by rectifying and smoothing an electric current from an AC supply is switched, a first switching element for conducting a switching operation in the PFC power supply section, a second switching element for conducting a switching operation in the DC—DC power supply section, a drive pulse generating circuit for generating first drive pulses for driving said first switching element and second drive pulses for driving said second switching element and a servo loop for controlling the drive pulse generating circuit.
Prior art solutions aim at maintaining a constant voltage Udc in the DC intermediate circuit by using a high-capacitance DC intermediate capacitor Cdc for intermediate energy storage. The ratings of the capacitors are generally determined by the capacitors' ability to withstand the electric current ripple and voltage loading applied to them and the required service lifetime. These requirements normally cause the DC capacitor components to be bulky and expensive.
Further, the line current in the supply AC mains in the prior art frequency converters is neither sinusoidal nor in phase with the supply voltage. For this reason the prior art single phase frequency converters can be provided with an active PFC (Power Correction Factor) circuit 15 in order to make the input current sinusoidal, and to compensate the power factor, so that the line current Iin will be in phase with the line voltage Uin. However, such PFC circuits make the frequency converter more expensive and complicated.
The object of the prior art frequency converter is to control the output voltage Uout so that the motor shaft torque is as smooth as possible. On the other hand it is known that in most applications where one phase motors have been used, e.g., in pump and fan drives, the shaft torque does not need to be smooth. This is obvious according to the one phase motor signal waveforms presented in FIG. 3, where u=line voltage, i=line current and P=motor power (P=u*i). Because both the voltage and current are sinusoidal, the power fed to the motor is sinusoidal also. Normally the load inertia is so high that the shaft speed remains about constant, which means that also the shaft torque fluctuation is similar to that of the power (P=ωT, where co is the shaft angular speed).